A refractory castable is generally defined as a combination of refractory grains and a suitable amount of a bonding agent that, after the addition of proper liquid, becomes rigid as a result of chemical action. Refractory castables generally include alumina aggregate together with a hydraulic-setting cement, and are classified based upon the amount of cement contained therein. For example, a conventional castable refractory will contain hydraulic-setting cement and have a total lime (CaO) content greater than 2.5% on a calcined basis. A low-cement castable refractory would have a total lime (CaO) content of between about 1% to 2.5% on a calcined basis. Ultra low cement castable refractories have a total lime (CaO) content of between about 0.2% to 1% on a calcined basis.
In recent years, the shotcreting of refractory castable has become a major method of installation of refractory linings in steel, non-ferrous metal, chemical, mineral and ceramic processing plants. Refractory castable shotcreting is performed by mixing a refractory material with water to provide a material having a pumpable consistency. After mixing, the refractory is delivered to a pumping device, such as a swing-valve pump, to convey the refractory through a delivery hose to a nozzle. At the nozzle, an accelerating admixture is added to the castable refractory to initiate setting of the refractory so that when sprayed on a wall or surface to be lined, the refractory does not slump or sag. The equipment required to shotcrete refractory castables is relatively expensive, and requires significantly more set-up and clean-up time as compared to a conventional dry gunning process.
Gunning is a technique wherein a special refractory "gunning mix" is transported through a delivery hose by pneumatic means to a nozzle where water (and possibly additional air) is (are) added to the refractory mix immediately prior to applying to a surface. Importantly, gunning mixes are generally granular refractory mixes specifically prepared for application with air displacement guns. A gun mix is made up of three basic ingredients: aggregate, clays and binders. The clays are added to the gun mix to increase stickiness and to prevent slumping of the refractory, reduce rebound during spraying and to expand the acceptable water ranges. Stated another way, clay is typically added to a gun mix to provide it with the physical properties which would enable it to be conveyed in a dry state through the delivery hose and to adhere to the surface to be lined when water is added immediately prior to spraying.
The ability to dry process gun a refractory greatly reduces the cost in equipment and set-up and clean-up time. A problem with such a process, however, is that by adding clay, or other like materials, to a refractory, the physical properties of the refractory are adversely affected. Basically, clay reduces the overall refractoriness of the material and provides a less desirous lining than could be achieved by a refractory castable.
The present invention overcomes the drawbacks inherent to each system and provides a method of applying a refractory castable by a dry gunning process.